Slayers are strong because they are the ultimate human/demon hybrid. Their stronger/more skilled than most vampires, and better trained/smarter than your average demon.
The things that out and out are shown being more powerful than a slayer are:
Adam: Unique human/demon/robot hybrid- one of a kind.
Glory: Hell-God. Also pretty unique.
Dark Willow: Uber-Witch with years of extreme effort, some natural talent and access to a couple of seriously powerful magical batteries on hand.
The Beast: Rock demon that defeated Angel and the entire Fang Gang (all well armed) without much effort. As well as defeating a slayer (out-of-practice, unarmed)
(Probably) Jasmine/Illyria: Higher power/Old One respectivly. Again pretty unique.
Everything else has (after maybe getting lucky once) been simply out-fought and had their asses kicked/gotten killed given enough time and circumstance; true demons, ancient vampires (The Master, Luke, Kakistos- some of the oldest on record), that order of assassins from S2, Mohra demon, Turok-Han, Caleb, ec.